Nonlinear optical effects are phenomena in which light entering a medium causes polarization that is proportional to higher-order terms of the square of electric field caused by the incident light. These phenomena lead to the generation of second harmonic waves, sum frequency waves, difference frequency waves, etc.
Those phenomena develop in materials generally called "nonlinear optical materials". Examples of known nonlinear optical materials include inorganic materials such as KH.sub.2 PO.sub.4, LiNbO.sub.3 and LiTaO.sub.3. It has recently been found that organic materials such as 2-methyl-4-nitrile-aniline (MNA), 4-dimethylamino-3-acetamidonitrobenzene (DAN) and 3,5-dimethyl-l-(4-nitrophenyl)pyrazole (DMNP) also have large nonlinear optical constants, and increasing attention is being paid to these organic nonlinear optical materials.
Various research efforts are made for applying the organic nonlinear optical materials to optical fiber light wavelength converter devices which are capable of reducing the wavelength of laser light by half. Devices using DAN or DMNP as the organic nonlinear optical material are known as such devices.
It is important to insure high-yield generation of second harmonics in those devices that they should be designed in such a way that the fundamental wave is confined at high energy density and that the interaction between the fundamental wave and harmonics is insures. To meet these requirements, either one of a core and a cladding of an optical fiber is formed of a single-crystal or polycrystalline nonlinear optical material and the other is formed of an amorphous material such as glass, and the fundamental wave is guided through the core, to achieve high conversion efficiency. FIG. 12 shows how the fundamental wave passing through core 101 of optical fiber light wavelength converter device 100 is converted to second harmonic 103 which emerges from the converter. Numeral 102 in FIG. 12 denotes a cladding of device 100.
Another requirement for the optical fiber light wavelength converter device is to insure matching of the propagation speed of the fundamental wave and that of the generated second harmonic, i.e., phase matching between them. The mechanism of the phase matching is illustrated in FIG. 13 on the assumption that light propagating through core 91 generates a second harmonic at point A, which leaks into cladding 92 at angle .alpha.. If the equiphase plane of this second harmonic agrees with the equiphase plane of another second harmonic that emits in the direction of angle .alpha. at point B after the passage of a unit time, second harmonics are radiated in the same direction of angle .alpha. (Cherenkov radiation). If the condition of the following equation is satisfied, phase matching is assured automatically to enable Cherenkov radiation: EQU n.sub.S (2.omega.)&gt;n.sub.G (.omega.)&gt;n.sub.S (.omega.)
wherein n.sub.S (.omega.) represents the refractive index of cladding 92 with respect to the fundamental wave; n.sub.G (.omega.) represents the refractive index of core 91 with respect to the fundamental wave; and n.sub.S (2.omega.) represents the refractive index of cladding 92 with respect to the second harmonic.
The optical fiber light wavelength converter device uses an organic crystal as the core. This has presented several problems such as evaporation of the crystal from the fiber-ends to lower the coupling efficiency of laser light, and moisture absorption by the crystal to increase guiding loss. JP-A-4-107509 (the term "JP-A" as used herein means an "unexamined Japanese patent application") proposes to solve these problems a method in that an optical fiber is cut in vacuum and a film of SiO.sub.2 or MgF.sub.2 is formed on the cut end faces, and JP-A-3-199524 propose a method in that the cut end faces of the fibers are coated with a polymer having the organic crystal of the core dissolved therein.
However, in the former method, since the organic crystal usually does not have good compatibility with the inorganic materials such as SiO.sub.2 and MgF.sub.2, the inorganic film over the organic crystal becomes porous or its thickness decreases to increase the chance of cracking. As a result, it is difficult to achieve complete prevention of the invasion of water which is a cause of device deterioration. In the later method which depends on the coating of a polymer having the organic crystal of the core dissolved therein, the core crystal dissolves or a crystal is deposited on the core crystal depending on temperature variations, whereby not only is the incident efficiency of laser light instabilized but also the device is deteriorated by water contained in the polymer.